This invention is generally related to analog signal processing and more particularly to techniques for extracting information from semiconductor storage arrays.
Semiconductor storage arrays are used to store and generate analog signals. For instance, a random access memory (RAM) has an array of storage cells in which one bit of information is stored. Another type of storage array is a sensor array that has cells which electrically respond to incident radiation. Image sensor arrays, for example, respond to light which forms an image of a scene on the array. The image array typically has a large number of photocells or pixels, where the projected image is recorded by analog signals generated by the pixels. An example of a modern complimentary metal oxide semiconductor (CMOS) sensor array is discussed in Article 1, Progress in CMOS Active Pixel Image Sensors, S. K. Mendis, S. E. Kemeny, R. C. Gee, B. Pain, Q. Kim and E. R. Fossum, SPIE, Vol. 2172, pages 19-29. Examples of imaging systems that use image arrays include traditional applications such as video cameras and copiers to more modern ones such as the facsimile machine, scanner, medical imaging device, and the digital camera. The Article 2, An 800K-Pixel Color CMOS Sensor for Consumer Still Cameras, J. E. D. Hurwitz, P. B. Denyer, D. J. Baxter, and G. Townsend, SPIE Vol. 3019, pages 115-124, describes an image sensor particularly suitable for digital cameras.
Modern imaging arrays can be very large, having as many as 1024xc3x971024 pixels, with future arrays expected to be even larger. The size is needed to provide detailed images. A 1024xc3x971024 array for instance generates over 1 million different analog signals to represent an image frame. The large number of signals thus presents the circuit designer with some problems which need to be addressed in order to improve overall imaging system performance. These include pixel signal readout speed and overall power consumption.
The readout speed is related to how fast the individual analog signals can be separated from all of the other signals produced by the array, passed through an analog transmission path, and fed to a signal processing pipe. The faster each signal pair is fed into the pipe, the greater the image frame rate. Greater image frame rate in turn facilitates capturing motion in the scene.
In addition to readout speed as an area of improvement, power consumption is also a major concern for at least two reasons. First, portable imaging systems such as modern digital cameras normally use batteries which have a limited source of energy. Thus, reducing power consumption in such systems extends battery life and leads to a more attractive consumer product. Second, imaging arrays and their associated readout circuitry are now being built on the same semiconductor die in order to reduce manufacturing costs. Examples include imaging arrays built using standard complimentary metal oxide semiconductor (CMOS) fabrication processes. When readout circuitry and pixels are built on the same die, the power dissipated by the readout circuitry heats the pixels. Heating the pixels in turn increases leakage currents within each pixel, and as a result changes pixel response, typically resulting in undesirably brighter and less accurate images. The problem becomes worse in larger arrays if the power dissipated by the readout circuitry is tied to the array size.
In view of the above, there is a need for a readout architecture and method to be used with imaging arrays which optimizes readout speed and helps minimize power consumption.
In a first embodiment, the invention features a semiconductor circuit having a storage array with a number of output lines. Each storage cell in the area generates first and second signals on a given output line. A sense amplifier array having a number of sense amplifier cells is coupled to the storage array, where each sense amp cell generates a differential signal pair in response to receiving the first and second signals on an output line.
The circuit also includes an analog multiplexer having first and second multiplexers (muxes), each mux having a number of inputs and one output, the first muxes receiving the differential signal pairs at their inputs. A subset of the first muxes are associated with a second mux. The inputs of the associated second mux are coupled to a number of outputs of the subset of the first muxes. Finally, control logic is provided for selecting a first mux from the subset and the associated second mux, to pass a single differential signal pair to be read out of the analog multiplexer.
In another embodiment, the sense amplifier cells are used to implement correlated double sampling (CDS) in an imaging apparatus. The sense amp cells form a row and are coupled to the bitlines of an image sensor array. An analog-to-digital converter unit is coupled to the analog mux for converting analog signals related to the differential signal pair into digital signals representing raw image data. A digital signal and image processing unit generates captured image data in response to receiving the digital signals. The captured image data is then transferred to a separate image processing system, such as a host computer. To orchestrate the events in the apparatus, a system controller is provided which can create the timing signals need for readout in response to instructions stored in firmware.